1. Field of Disclosure
The disclosure relates to high power fiber laser systems. In particular, the disclosure relates to a clad mode stripper/absorber operative to trap and remove undesirable cladding-guided light in passive fibers.
2. Prior Art
High power single mode fiber laser systems (“HPSMFLS”) are typically configured with one or more amplifying cascades each including an active double clad fiber which is either side- or end pumped by multimode (MM) pump light. The pump light propagating along a light-guide waveguide cladding is often not fully absorbed along the length of the active fiber and eventually coupled into a cladding of SM passive fibers. In addition, power losses at splices are also responsible for multimode light which propagates in a waveguiding cladding under a polymeric protective sheath. Besides, backreflecting light from the surface to be laser treated may also be coupled in the cladding. Even at moderate pump light absorption rates varying between 10-12 dB the clad-guided light can easily reach 300-600 W at the final amplifying stage. The clad guided light is undesirable for the following reasons.
Typically, the SM passive fiber of HPSMFLS is surrounded by a flexible polymeric protective sheath which has a lower refractive index than that of the cladding region. At fiber bends or spliced connections, the total reflection of the cladding region may be disturbed leading to the escape of clad-guided light into the protective sheath. As a consequence, the protective sheath may be overheated and destroyed. Furthermore, the MM radiation carried in the cladding may damage the end regions of the fiber which is typically a passive, SM delivery fiber or SM passive fiber coupled between two gain blocks, one of which, for example, is pumped in a direction counter to the signal propagation. Finally, if the clad-guided MM radiation reaches the end of the delivery fiber, the beam quality of the transmitted light signal may be worsened, which can adversely affect other optical components and the processing quality.
Devices configured to remove clad light and convert the light energy into the heat energy are known as cladding mode absorbers (“CMA”). Typically, a CMA is provided along a length of fiber stripped from the protective sheath, which covers the cladding, and configured as a light-guide polymer compound with a refractive index higher than that one of the cladding. The polymer compound absorbers typically allow decoupling cladding light with a power up to about 100 to about 400 W.
Some of structural limitations of the known CMAs include a low resistance to mechanical stresses caused by non-uniform thermo-dependent expansion/contraction of fiber and CMA. Typically, mechanical stresses lead to micro bending losses in single mode (“SM”) large mode area (“LMA”) fibers and excitation of high order modes (“HOMs”).
Still a further limitation is associated with a relatively low thermal conductivity of polymer compound leading to its high temperatures. A 110° C. temperature is generally considered critical, and higher temperatures lead to the destruction the CMA. This critical temperature can be even lower during abrupt temperature changes occurring upon energizing a fiber laser system.
A need therefore exists for a HPSMFLS configured with a CMA which is characterized by high temperature- and mechanical stress-resistant structure.
A further need exists for a method of manufacturing the improved CMA.